Daryl Lindsay Nielsen

Dominant ecological paradigms provide theoretical frameworks for two basal sources of carbon to riverine food webs: carbon fixed by algae and submerged macrophytes (autochthonous) and carbon fixed by terrestrial vascular plants that enters aquatic ecosystems as litter (allochthonous). An additional, less‐studied, pathway for carbon to enter rivers is as methane that is produced in small quantities via methanogenesis by anaerobic methanogenic archaea. Because rivers are generally well‐oxygenated, methane is not often recognised as a substantial contributor to riverine carbon budgets. However, methane may also enter rivers via natural gas macroseeps where underground deposits connect to the surface via fissures and fractures. We explored the contribution of methane from natural gas macroseeps to food webs by comparing δ¹³C values of animals and their foods from reaches rich in dissolved methane with reaches where methane concentrations were at background levels. Animals collected from river reaches with natural gas macroseeps had significantly depleted δ¹³C values compared to animals from reference reaches. Within natural gas macroseep reaches, methane served as the principal basal carbon source for mayfly primary consumers (Tasmanocoenis spp.), contributing up to 55% of their total carbon budget. Methane‐derived carbon persisted across multiple trophic levels of the natural gas macroseep food web, subsidising up to 19% of the carbon in Decapoda and 28% in carnivorous fish. Among Decapoda, carbon of terrestrial origin dominated biomass contributions, supplying up to 55% of carbon to Macrobrachium australiense and 56% to Parataya australiensis. Autochthonous carbon contributions were highest among mayflies, with up to 30% of their carbon fixed by algae. Our findings establish the prevalence of a natural gas‐derived methane–methanotrophic bacteria–primary consumer pathway. We emphasise its significance as a potentially dominating energy conduit in rivers with high concentrations of dissolved methane, demonstrating feeding preference redundancy at low trophic levels. Additionally, we provide useful information for the incorporation of different sources of carbon in riverine food webs. Read the free Plain Language Summary for this article on the Journal blog.

Microcrustacea in ephemeral wetlands produce dormant eggs to escape prolonged dry conditions. These eggs can hatch on inundation, although in most cases not all eggs hatch during a single wetting event. Incomplete hatching can reflect bet-hedging strategies, but also the presence or absence of environmental cues that stimulate hatching. This study examines the effects of environmental cues likely to change for wetlands in the future, namely, temperature and water depth. Surface sediments collected from dry anabranches of the Macintyre River floodplain (eastern Australia) were inundated under two temperature regimes (warm and cool) in microcosms of two depths (shallow and deep). Hatched microcrustacea were sampled for 6 weeks. The abundance and assemblage composition of microcrustacea varied by temperature but not by depth. Although the total abundance was greater under warm conditions, the effect of temperature diminished over time. Temperature also had a greater effect on non-ostracods, with 144% more non-ostracods being hatched under warm than under cool conditions. Thus, changes to temperature during inundation periods arising from global climate change or river regulation are likely to influence the abundance and composition of microcrustacean assemblages, especially among non-ostracods, which will influence food availability for larval and juvenile native fish and, hence, recruitment.

Cladocerans are important filter-feeders transferring energy up the food web to different invertebrate and vertebrate predators. Along the flood period, cladocerans are one of the primary food sources for juvenile fish in floodplain. Resting egg banks allow cladoceran populations to overcome the environmental stress, related to several limnological changes, including complete drying of temporary wetlands. After drought, resting egg banks influence cladoceran community attributes during the cyclic and successional processes driven by episodic flood events. In this study we compared the taxonomic richness of active (from the water column) and dormant (from the sediment) Cladocera assemblages and analyzed the structure of resting egg banks, comparing the diversity, abundance and apparent viability/unviability of the eggs, between six temporary and six permanent wetlands, located along the Ovens River Floodplain, Victoria, Australia. The qualitative analysis shows higher taxonomic richness in active assemblages from temporary (24 taxa) than permanent (13 taxa) wetlands compared to dormant assemblages present in resting egg banks (9 taxa) from temporary and permanent wetlands. However, richness was influenced by taxonomic level of identification, with the majority of resting eggs only being identified to the taxonomic level of family (i.e. Chydoridae). Total taxa richness within egg banks was similar between wetland types, however, on average higher Shannon’s diversity of resting eggs was found within permanent (1.53) than temporary (0.82) wetlands. This is likely to be due to more stable wetlands not providing appropriate cues to trigger dormancy induction or breakage for specific populations, leading to higher values of evenness in permanent than temporary wetlands. Comparing permanent and temporary wetlands, higher abundance of resting eggs (more than four times) consisting of higher abundance of unviable eggs and similar viable egg abundance to permanent wetlands, was found within temporary wetlands, suggesting that the increased resting egg abundance in temporary wetlands is balanced by the losses due to factors such as predation, parasitism or other physical damage, during the terrestrial phase. Despite resistant outer shell structure, this study highlights that the damage to egg integrity is intensified in wetlands that undergo dry phases. Cladoceran resting egg banks represent the potential assemblage to recover after disturbance events such as drying, and information about these is important to ensure appropriate management and conservation of floodplain biodiversity.

Wetlands are one of the most threatened ecosystems on earth. Globally, there is growing demand to develop infrastructure to harvest water resources to support agriculture, threatening the ecological integrity of associated wetland ecosystems. We investigated the potential ecological impacts on floodplain wetland connectivity in northern Australia in response to changes in flow regime due to dam construction and climate change. Results for this study indicate that a drier climate and/or dam construction in catchments have the potential to reduce the effective size of the floodplain and reduce wetland connectivity. A drier climate will reduce rainfall and subsequent catchment run‐off, resulting in a decrease in the magnitude of riverine flows. In contrast, dams in the upper catchment will reduce the magnitude of flows downstream. The reduction in flows under both dam development and a drier climate will result in reduced extent and duration of floodplain inundation and decreased wetland connectivity. As a result, we anticipate that this loss of connectivity will reduce the capacity for nutrients, carbon and primary production to be flushed into the river channel, as well as reduce the ability for biota such as fish and turtles to move between in‐channel and off‐channel habitats.

Future climate change predictions indicate that there will be an increase in ambient air temperature. Increases in ambient air temperature will result in a corresponding increase in soil temperature. The consequences of further increases in soil temperature will potentially be detrimental for the soil seed bank of plants in terms of length of dormancy and viability of seeds. This experiment investigated the effect of different exposure temperatures and duration of exposure on the germination of semi-aquatic plant species. Seeds of four species (Alternanthera denticulata, Juncus usitatus, Persicaria lapathifolia and Persicaria prostrata) were exposed to temperatures ranging from 25 to 100°C for durations between 1 and 14 days, before being germinated in an incubator for 6 weeks. Germination occurred in all four species after exposure to temperatures ranging from 25 to 60°C. These temperatures appeared to promote germination as the temperature and duration of exposure increased. However, in P. lapathifolia and P. prostrata, the number of seeds germinating declined when exposed to 70°C and there was no germination for temperatures exceeding this. In contrast, A. denticulata and J. usitatus only began to decline when exposed to 80°C, with no germination at higher temperatures. These results suggest that soil temperatures exceeding potential threshold temperatures of 70 and 80°C will result in a decline in the number of seeds germinating and may potentially see a change in species distributions. As such soil temperatures are already being experienced throughout Australia, some species may already be close to their thermal threshold.